9000
J. Am. Chem. Soc. 2001, 123, 9000-9006
Olefin Metathesis in Supercritical Carbon Dioxide
Alois Fu1rstner,* Lutz Ackermann, Karsten Beck, Hisao Hori, Daniel Koch,
Klaus Langemann, Monika Liebl, Christian Six, and Walter Leitner*
Contribution from the Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1,
45470 Mu¨lheim/Ruhr, Germany
ReceiVed April 12, 2001
Abstract: Liquid or supercritical carbon dioxide (scCO2) is a versatile reaction medium for ring-opening
metathesis polymerization (ROMP) and ring-closing olefin metathesis (RCM) reactions using well-defined
metal catalysts. The molybdenum alkylidene complex 1 and ruthenium carbenes 2 and 3 bearing PCy3 or
N-heterocyclic carbene ligands, respectively, can be used and are found to exhibit efficiency similar to that in
chlorinated organic solvents. While compound 1 is readily soluble in scCO2, complexes 2 and 3 behave like
heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO2 provide
significant advantages beyond simple solvent replacement. This pertains to highly convenient workup procedures
both for polymeric and low molecular weight products, to catalyst immobilization, to reaction tuning by density
control (RCM versus acyclic diene metathesis polymerization), and to applications of scCO2 as a protective
medium for basic amine functions. The latter phenomenon is explained by the reversible formation of the
1
corresponding carbamic acid as evidenced by H NMR data obtained in compressed CO2. Together with its
environmentally and toxicologically benign character, these unique physicochemical features sum up to a very
attractive solvent profile of carbon dioxide for sustainable synthesis and production.
Introduction
cal interactions of scCO2 with substrates and/or catalysts offer
an as yet largely unexplored potential for activity and selectivity
control.8 The utilization of scCO2 as solvent and C1-building
block opens further opportunities for homogeneous catalysis.4,8,9
Olefin metathesis refers to the mutual alkylidene exchange
reaction of alkenes.10 This type of reaction has become
increasingly important as a synthetic tool during the last few
years owing to the application of well-defined carbene com-
plexes as catalysts or catalyst precursors.11 Herein we report
on transition-metal-catalyzed olefin metathesis in compressed
(supercritical or liquid) CO2.12 As shown below, the reaction
can be performed efficiently in this unconventional medium,
and even more intriguingly, the properties of supercritical CO2
remarkably increase the scope of this versatile transformation.
Detailed investigations provide insights into mechanistic aspects
and therefore help to rationalize the beneficial effects at the
molecular level.
Numerous advantages are associated with the use of com-
pressed and especially supercritical carbon dioxide (scCO2;
Tc ) 31.0 °C, pc ) 73.8 bar, dc ) 0.477 g‚mL-1) as an
ecologically benign and economically feasible reaction medium
for metal-catalyzed reactions.1,2 Technical aspects such as its
nonflammability and the lack of virtually any toxicity and
immediate ecological hazards make scCO2 particularly attractive
for sustainable chemical processes.3 The high miscibility of
scCO2 with many reaction gases and the absence of a gas/liquid-
phase boundary can lead to enhanced reaction rates.4 The rich
phase behavior of supercritical reaction media provides novel
approaches to catalyst immobilization.5 In addition to solubility
control, other physiochemical properties of a supercritical fluid
can also be tuned within a certain range by adjusting its density
through small variations of pressure and temperature.6,7 Chemi-
* To whom correspondence should be addressed. E-mail: (A.F.)
fuerstner@mpi-muelheim.mpg.de, (W.L.) leitner@mpi-muelheim.mpg.de.
(1) Chemical Synthesis Using Supercritical Fluids; Jessop, P. G., Leitner,
W., Eds.; Wiley-VCH: Weinheim, 1999.
(6) Wynne, D. C.; Olmstead, M. M.; Jessop, P. G. J. Am. Chem. Soc.
2000, 122, 7638-7647.
(7) For a mathematical discussion of the specific solvent effects in
supercritical fluids see: Clifford, A. In ref 1, pp 54-66.
(8) For the coordination chemistry of CO2 and its relevance for catalysis
see: (a) Leitner, W. Angew. Chem. 1995, 107, 2391-2405; Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2207-2221. (b) Leitner, W. Coord. Chem. ReV.
1996, 153, 257-284.
(2) For selected reviews, see: (a) Jessop, P. G.; Ikariya, T.; Noyori, R.
Science (Washington, D.C.) 1995, 269, 1065-1069. (b) Morgenstern, D.
A.; LeLacheur, R. M.; Morita, D. K.; Borkowsky, S. L.; Feng, S.; Brown,
G. H.; Luan, L.; Gross, M. F.; Burk, M. J.; Tumas, W. In Green Chemistry;
Anastas, P. T., Williamson, T. C., Eds.; ACS Symposium Series, Vol. 626;
American Chemical Society: Washington, DC, 1996; pp 132-151. (c)
Dinjus, E.; Fornika, R.; Scholz, M. In Chemistry under Extreme or Non-
Classical Conditions; van Eldick, R., Hubbard, C. D., Eds.; Wiley: New
York, 1997; pp 219-271. (d) Jessop, P. G.; Ikariya, T.; Noyori, R. Chem.
ReV. (Washington, D.C.) 1999, 99, 475-493. (e) Leitner, W. Top. Curr.
Chem. 1999, 206, 107-132.
(3) Wells, S. L.; DeSimone, J. M. Angew. Chem. 2001, 113, 534-544;
Angew. Chem., Int. Ed. 2001, 40, 518-527.
(4) Jessop, P. G.; Ikariya, T.; Noyori, R. Nature (London) 1994, 368,
231-233.
(5) (a) Francio`, G.; Wittmann, K.; Leitner, W. J. Organomet. Chem. 2001,
621, 130-142. (b) Kainz, S.; Brinkmann, A.; Leitner, W.; Pfaltz, A. J.
Am. Chem. Soc. 1999, 121, 6421-6429. (c) Koch, D.; Leitner, W. J. Am.
Chem. Soc. 1998, 120, 13398-13404.
(9) (a) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. J. Am. Chem.
Soc. 1996, 118, 344-355. (b) Reetz, M. T.; Ko¨nen, W.; Strack, T. Chimia
1993, 47, 493.
(10) Ivin, K. J.; Mol, J. C. Olefin Metathesis and Metathesis Polymer-
ization; Academic Press: San Diego, 1997.
(11) Reviews: (a) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001,
34, 18-29. (b) Fu¨rstner, A. Angew. Chem. 2000, 112, 3140-3172; Angew.
Chem., Int. Ed. 2000, 39, 3012-3043. (c) Grubbs, R. H.; Chang, S.
Tetrahedron 1998, 54, 4413-4450. (d) Fu¨rstner, A. Top. Catal. 1997, 4,
285-299. (e) Schuster, M.; Blechert, S. Angew. Chem. 1997, 109, 2124-
2144; Angew. Chem., Int. Ed. Engl. 1997, 36, 2037-2056.
(12) For a preliminary report see: Fu¨rstner, A.; Koch, D.; Langemann,
K.; Leitner, W.; Six, C. Angew. Chem. 1997, 109, 2562-2565; Angew.
Chem., Int. Ed. Engl. 1997, 36, 2466-2469.
10.1021/ja010952k CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/23/2001